A liquid crystal display (LCD) has advantages of portability, low power consumption, and low radiation. LCDs have been widely used in various portable information products, such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCDs are considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, an inverter circuit for driving the backlight, and a backlight control circuit. The backlight control circuit generally includes a pulse width modulation integrated circuit (PWM IC) for driving the inverter circuit, and a backlight protection circuit for shutting down the PWM IC when any lamp of the backlight has an open circuit or a short circuit.
FIG. 2 is a circuit diagram of a typical backlight control circuit. The backlight control circuit 100 includes four load circuits 110, a PWM IC 150, and a backlight protection circuit (not labeled). The backlight protection circuit includes an input circuit 130 and a switch circuit 170.
Each load circuit 110 includes a lamp 111 and a lamp inspecting circuit 113 connected in series between a power supply (not shown) and ground. The lamp inspecting circuit 113 includes an output terminal 112. The output terminal 112 provides a high voltage when the corresponding lamp 111 works, and provides a low voltage when the corresponding lamp 111 has an open circuit or a short circuit.
The PWM IC 150 includes a current sampling pin 151. The PWM IC 150 stops working if the current sampling pin 151 has a low voltage.
The switch circuit 170 includes a first transistor 1331, a first current limiting resistor 172, a second current limiting resistor 173, and an output terminal 175. A source electrode of the first transistor 1331 is grounded, and the drain electrode of the first transistor 1331 is connected to the current sampling pin 151 of the PWM IC 150. A gate electrode of the first transistor 1331 is connected to a power supply VDD via the first current limiting resistor 172, and a value of the power supply VDD is 5V (volts). The drain electrode of the first transistor 1331 is also connected to the output terminal 175 via the second current limiting resistor 173, and the output terminal 175 is connected to a high level signal source (not shown).
The input circuit 130 includes four diodes 131, four resistors 132, four capacitors 135, a second transistor 1332, a third transistor 1333, a fourth transistor 1334, and a fifth transistor 1335. A drain electrode of the second transistor 1332 is connected to the gate electrode of the first transistor 1331. A drain electrode of the third transistor 1333 is connected to a source electrode of the second transistor 1332. A drain electrode of the fourth transistor 1334 is connected to a source electrode of the third transistor 1333. A drain electrode of the fifth transistor 1335 is connected to a source electrode of the fourth transistor 1334. A source electrode of the fifth transistor 1335 is grounded. Gate electrodes of the second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 are connected to cathodes of the four diodes 131, respectively. Anodes of the four diodes 131 are respectively connected to a corresponding output terminal 112 of the lamp inspecting circuit 113. Each of the gate electrodes of the transistors 1332, 1333, 1334, 1335 is grounded via a corresponding resistor 132 and via a corresponding capacitor 135, respectively.
The first transistor 1331, the second transistor 1332, the third transistor 1333, the fourth transistor 1334, and the fifth transistor 1335 are all negative-channel metal oxide semiconductor (NMOS) type transistors.
Operation of the backlight control circuit 100 is as follows. When all the lamps 111 work normally, each of the output terminals 112 provides a high voltage to the gate electrode of the corresponding second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 via the corresponding diode 131. Then the second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 are switched to an activated state, and the gate electrode of the first transistor 1331 is grounded via the activated second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335. Thus, the first transistor 1331 is turned off. Because the output terminal 175 of the switch circuit 170 is connected to the high level signal source, the current sampling pin 151 of the PWM IC 150 receives a high level signal, and the PWM IC 150 operates normally.
When any one of the lamps 111 has an open circuit or a short circuit, the corresponding output terminal 112 provides a low voltage to the gate electrode of the corresponding second, third, fourth, or fifth transistor 1332, 1333, 1334, or 1335 via the corresponding diode 131. Then the corresponding second, third, fourth, or fifth transistor 1332, 1333, 1334, or 1335 is turned off, so that the gate electrode of the first transistor 1331 is charged to a high voltage by the power supply VDD via the first current limiting resistor 172. Thus, the first transistor 1331 is switched to an activated state, and the current sampling pin 151 of the PWM IC 150 is grounded via the activated first transistor 1331. Consequently, the current sampling pin 151 of the PWM IC 150 is charged to a low voltage, and the PWM IC 150 stops working.
The backlight control circuit 100 includes five transistors 1331, 1332, 1333, 1334, and 1335 to carry out the function of protecting the lamps 111. Furthermore, the number of the transistors needs to increase along with the number of lamps 111 used in the LCD. Consequently, the cost of the backlight control circuit 100 is high, particularly in the case where the number of lamps 111 is large.
It is desired to provide a backlight control circuit used typically in an LCD which overcomes the above-described deficiencies.